There are a number of different circumstances in which it is desirable to perform analysis to identify and/or measure compounds in a sample. Such samples may be taken directly from the environment or they may be provided by front end specialized devices to separate or prepare compounds before analysis. There exists, a demand for low cost, compact, low-power, accurate, easy to use, and reliable devices capable of detecting compounds in a sample.
One class of known analyzers are gas chromatographs (GC). Gas chromatography is a chemical compound separation method in which a discrete gas sample (composed of a mixture of chemical components) is introduced via an injector arrangement into a GC column. Components of the introduced analyte sample are partitioned between two phases: one phase is a stationary bed with a large surface area, and the other is a gas phase which passes through, or past, the stationary bed. The sample is introduced into the mobile gas phase carrier gas (CG) and carried through the column. The sample partitions (equilibrates) into the stationary phase (often liquid), based on its solubility into the stationary phase material and the temperature of the column. The components of the sample separate from one another based on their relative vapor pressures and affinities for the stationary beds which causes the different compounds to be retained in the GC column for differing amounts of time.
Compounds can be identified by the amount of time they are retained within the GC column. The retention or elusion time (i.e., the time that a compound is retained within the GC column) is typically measured as the time from sample injection into the GC column to the time that a peak concentration/intensity for the compound is measured at a detector.
The prior art teaches two general types of GC columns, packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with the liquid stationary phase. Packed columns are typically between about 1.5 meters-about 10 meters in length and have an internal diameter of between about 2 millimeters-about 4 millimeters. Capillary columns typically have an internal diameter of a about a few tenths of a millimeter. They are typically either wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns have a capillary tube whose walls are coated with the liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material, such as diatomaceous earth, onto which the stationary phase is adsorbed. SCOT columns are generally less efficient than WCOT columns. Both types of capillary column are more efficient than packed columns.
Ideally, column temperature is controlled to within tenths of a degree. The optimum column temperature is dependant upon the boiling point of the sample. Generally, a temperature slightly above the average boiling point of the sample results in an elution time of 2-30 minutes. Lower temperatures give good resolution, but increase elution times. If a sample has a wide boiling range, then temperature programming can be useful. The column temperature is increased (either continuously or in steps) as separation proceeds.
There are many detectors that can be used with a GC providing different levels of selectivity. For example, a non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property, and a specific detector responds to a single chemical compound. Exemplary detectors include, flame ionization detectors (FID), thermal conductivity detectors (TCD), electron capture detectors (ECD), nitrogen-phosphorus detectors, flame photometric detectors (FPD), photo-ionization detectors (PID) and hall electrolytic conductivity detectors.
Certain components of high speed or portable GC analyzers have reached advanced stages of refinement. These include improved columns and sample injectors, and heaters that achieve precise temperature control of the column. Even so, detectors for portable GCs, generally thermal conductivity based, still suffer from relatively poor detection limits and selectivity. In addition, GC analyzers combined with conventional detectors, such as those mentioned above, produce a signal indicating the presence of a compound eluted from the GC column. However, presence indication alone is often inadequate. It is often desirable to obtain additional specific information about the analyte to enhance compound identification and reduce false positives and negatives.
One conventional approach for obtaining additional information combines a GC with a MS. Mass spectrometers are generally recognized as being the most accurate type of analyzers for compound identification. An advantage of employing a MS with a GC is that the MS provides an orthogonal set of information, based on molecular weight and charge, for each chromatographic peak of the GC. As used herein, the term “orthogonal” means data that is obtained by measuring a different property of the compound during sample analysis to provide multiple levels of relatively independent and accurate information. By providing orthogonal data, use of a MS as the detector increases the accuracy of analytical separation provided by the GC, and in most cases, the combined GC-MS information is sufficient for unambiguous identification of the compound. Unfortunately, the GC-MS is not well suited for portable field-deployable instruments, which need to be small and are desirably low cost. While GC's are continuously being miniaturized and reduced in cost, mass spectrometers are still very expensive, often exceeding $100 k. Mass spectrometers also suffer from other shortcomings, such as the need to operate at relatively low pressures, resulting in complex support systems. They also need a highly trained operator to tend to and interpret the results. Accordingly, mass spectrometers are generally difficult to use outside of laboratories.
Time-of-flight Ion Mobility Spectrometers (TOF-IMS) have also been employed as detectors for GCs, and exhibit functional parallels to MSs. However, despite advances over the past decade, TOF-IMS drift tubes as detectors for GCs have not been widely adopted. For good analytical performance, IMSs must be comparatively large as they suffer from losses in resolution when made small. Thus, field-deployment still remains difficult for GC-TOF-IMSs.
A class of chemical analysis instruments more suitable for field operation is known as Field Asymmetric Ion Mobility Spectrometers (FAIMS) or Differential Mobility Spectrometers (DMS), and also known as Radio Frequency Ion Mobility Spectrometers (RFIMS) among other names. Hereinafter, FAIMS, DMS, and RFIMS, are referred to collectively as DMS.
The DMS filtering technique involves passing ions in a drift gas through strong electric fields between filter electrodes. The fields are created by application of an asymmetric period voltage (typically along with a compensation voltage) to the filter electrodes. The process achieves a filtering effect by accentuating differences in ion mobility. The asymmetric field alternates between a high and low field strength condition, which causes the ions to move in response to the field according to their mobility. Typically, the mobility in the high field differs from that of the low field. That mobility difference produces a net displacement of the ions as they travel in the gas flow through the filter. In the presence of a specific compensation voltage, a particular ion species passes through the filter. The amount of change in mobility in response to the asymmetric field is compound-dependent. This permits separation of ions from each other according to their species, in the presence of an appropriately set compensation field bias.
Fast detection is a sought-after feature of a field deployable detection device. One characteristic of known DMS devices is the relatively slow detection time. However, the GC can operate much more rapidly, such that the known DMS devices cannot generate a complete spectra of the ions present under each GC peak. Therefore, conventional DMS devices are limited to a single compound detection mode if coupled to a GC, with a response time typically of about 10 seconds. Any additional compound that is desired to be measured takes approximately an additional 10 seconds to measure.
While the foregoing arrangements are adequate for a number of applications, there is still a need for a small, field-deployable sample analyzer that can detect certain ion species or discriminate certain species from others in an efficient manner.